Optical fibers are of great importance in many fields of science and technology. There are various parameters of optical fibers affecting their properties; one of the most important is the number of modes capable of non-attenuated propagation. Fibers carrying only a single transverse mode are called single mode (SM) fibers. Fibers capable of propagating more than one mode are called multimode fibers (MM). Different fiber modes have different propagation properties, e.g. group velocity, spatial intensity and phase profiles. Modal group velocity dependence in MM fibers affects the fiber's ability to transmit data along longer distances. Intensity and phase dependence affect the beam quality factor, i.e. the M2 parameter, of the beam exiting the fiber. This is important in laser application of fibers, when the laser beam quality degrades due to higher fiber modes.
Modal structure of the beam propagating in optical fiber depends on the beam intensity and phase profile at the fiber input. Thus by altering the beam properties at fiber input (W. Q. Thornburg et al., “Selective launching of higher-order modes into an optical fiber with an optical phase shifter”. Optics Letters, Vol. 19, No. 7, 1994, pgs. 454-456) or output, one can change the modal structure of the exiting beam. A common technique for controlling the phase of an optical beam is adaptive optics (Michael Bass—Editor in chief, Handbook of optics, Vol. 3, 2nd edition, 2001, pages 1.4-1.11). Adaptive optics is widely used in astronomical observations, where in order to achieve high resolution images, light phase should be dynamically compensated due to atmospheric turbulence distortions. Adaptive optics technology is based on sensing the incident beam phase by phase sensors and subsequent correction by an array of dynamic elements capable of altering the phase profile of the beam (H. Babcock, “The possibility of compensating astronomical seeing”, Publications of the Astronomical Society of the Pacific 65: 229-236, October, 1953). Phase altering arrays are based on piezoelectric actuated membrane mirrors or liquid crystal based phase masks. Adaptive optics technology has already made great impact in astronomical telescopes (F. Roddier, “Adaptive Optics in Astronomy”, Cambridge University Press, Cambridge, England, 1999, pages 1-9), retinal imaging systems (E. N. Ribak et al., “Speckle reduction in ocular wave front sensing”, Adaptive Optics: Analysis and Methods, OSA, Ed. B, Ellerbroek, Vancouver, Canada, June, 2007) and free space laser communication systems. Several researchers also applied liquid crystal phase mask based adaptive optics to compensate modal dispersion in MM fibers to improve communication bandwidth at longer fiber lengths (X. Shen et al., “Compensation for multimode fiber dispersion by adaptive optics”, Optics Letters, Vol. 30, No. 22, 2005, pgs. 2985-2987).
Fiber lasers are another example of important optical fiber applications, wherein the modal structure is of great importance (G. P. Agrawal, “Applications of non-linear fiber optics”, Academic Press, 2001, pages 201-250). For many years the development of fiber-coupled and active fiber lasers capable of maintaining high energy densities along with high beam quality has been a great challenge. Due to nonlinear processes and other thermal effects in fibers, parasitic modes are developed; the beam at the output of the fiber is then distorted, resulting in a reduction of beam quality.
The ability to dynamically control the modal structure of light in a fiber will greatly contribute to improvement of beam quality in these types of lasers, presently limited by optimization of static fiber properties only (for example large mode area and photonic crystal fibers—V. Laude et al., “Photonic band-gap guidance of acoustic modes in photonic crystal fibers”, Phys. Rev B 71, 045107, 2005).
It is therefore clear that it would be greatly desirable to provide a method that obviates the disadvantages of the prior art thus improving the beam quality.
It is an object of the present invention to provide such a method, which exploits adaptive optics techniques to alter the structure of light propagating in optical fiber.
It is another object of the invention to provide a method that is simple to implement, using existing equipment, with relatively low cost as compared with the advantages achieved.
Other objects and advantages of the invention will become apparent through the following description.